The present embodiments relate to positron emission tomography (PET). In particular, supplemental transmission information is used in PET for attenuation correction.
PET imaging may be combined with another imaging modality in a hybrid system. One example is a combination of PET with magnetic resonance (MR). The axial fields of view (FOV) of the individual modalities are as close together as possible in order to minimize the impact of patient motion and increase correlation of the respective data sets. In order for the MR and PET fields of view to overlap, the PET detectors are placed within the MR's axial field of view, as an insert between the MR gradient and radio-frequency body coils.
A hybrid PET/MR system provides benefits for PET imaging. The MR portion may be used to assist in the attenuation correction of PET imaging data. However, MR-based human attenuation correction for PET on integrated PET/MR systems remains problematic. The undistorted MR transaxial field of view at about 45 cm is smaller than the PET trans-axial FOV and tunnel diameter, 60 cm. The magnetic field for MR may have spatial distortions or not be uniform outside of the 45 cm radius. This leads to truncation of the patient's body, particularly the arms. Arm truncation causes serious quantitative errors in both attenuation correction and scatter correction of the PET data. In addition, the process of converting an MR image to PET attenuation correction coefficients involves segmenting the MR image into different tissue classes and then assigning assumed linear attenuation coefficient values to these tissues. However, the MR signal frequently does not differentiate clearly between certain tissue types, such as bone and lung tissue, that have very different PET attenuation characteristics. This can result in distortions and inaccuracies in the MR-derived image of PET linear attenuation coefficients, even within the MR FOV.
In one approach, a maximum likelihood for attenuation and activity (MLAA) algorithm uses the PET emission data itself to provide an estimate of a linear attenuation coefficient (LAC) image in the truncated region. However, the shapes and LAC values of the MLAA-estimated arms may be inaccurate, and MLAA may even fail entirely when there is minimal non-specific uptake in the body and arms.